1. Field of the Invention
The present invention relates to an exposure apparatus, which is used to expose fine patterns in a manufacturing process of semiconductor integrated circuits, etc., and a mask unit used therefor, and more particularly to an electron beam proximity exposure apparatus, in which a mask having apertures corresponding to a pattern to be exposed is disposed in proximity to a surface of an object such as a semiconductor wafer, the mask is irradiated with an electron beam, and exposure of the pattern with the electron beam having passed through the apertures is thereby performed, and a mask unit used therefor.
2. Description of the Related Art
Attempts are being made to enhance integration degrees of semiconductor integrated circuits and finer circuit patterns are desired. Presently, a limit of the finer circuit patterns is defined mainly by exposure apparatuses, and a stepper, which is an optical exposure apparatus, takes various measures such as a light source that emits rays having shorter wavelengths, a larger NA (numerical aperture) and a phase shift method. However, much finer circuit patterns involve various kinds of problems such as a rapid increase of manufacturing costs. New types of exposure apparatus such as an electron beam direct lithography apparatus and an X-ray exposure apparatus have been therefore developed, but there still remain many problems in terms of stability, productivity, cost, etc.
An electron beam proximity exposure system is conventionally under research and development, since the exposure principle thereof is simple, as “High Throughput Submicron Lithography with Electron Beam Proximity Printing” (H. Bohlen et al., Solid State Technology, September 1984, pp. 210-217) (hereinafter referred to as a literature 1) exemplifies. However, it was thought that it was of no practical use since it was difficult to eliminate the proximity effect peculiar to the electron beam.
U.S. Pat. No. 5,831,272 (corresponding to Japanese Patent No. 2951947) and “Low energy electron-beam proximity projection lithography: Discovery of missing link” (Takao Utsumi, J. Vac. Sci. Technol. B 17(6), November/December 1999, pp. 2897-2902) disclose an electron beam proximity exposure apparatus that overcomes the above-mentioned problems and is usable for processing with very fine resolution at a mass production level.
FIG. 1 is a view showing a fundamental configuration to realize the electron beam proximity exposure apparatus disclosed in U.S. Pat. No. 5,831,272. As shown in FIG. 1, in an electron optical column 10 are disposed an electron gun 14, which emits an electron beam 15, a condenser lens 18, which collimates the electron beam 15, a main deflecting device 20 and a subsidiary deflecting device 50. Although the main deflecting device 20 is shown as a single deflecting device in FIG. 1, it is actually configured in two stages so as to obtain electron beams that are in parallel with an optical axis and have different irradiating locations by deflecting an electron beam with a deflecting device in the first stage and then in a reverse amount with a deflecting device in the second stage. Similarly, the subsidiary deflecting device 50 is also configured actually in two stages so that fine adjustment of an irradiating angle is possible without changing the irradiating locations changed with the main deflecting devices by deflecting the electron beams with a deflecting device in the first stage and then in a reverse amount twice as large with a deflecting device in the second stage. In a vacuum object chamber 8 are disposed a mask stage 36, which holds and moves a mask 30, a reflected electron detector 38, which detects reflected electrons, a wafer stage 44, which holds and moves a wafer 40, a standard mark 60 disposed on the wafer stage 44, and a height measurer 46, which measures a height of the surface of the wafer 40. A laser length measuring device 38 for the mask stage, which measures a travel amount of the mask stage 36, and a laser length measuring device 48 for the wafer stage, which measures a travel amount of the wafer stage 44, are disposed so that the travel amounts of the stages can be measured with remarkably high accuracy. The wafer stage 44 is movable in directions of at least two axes. Although the reflected electron detector 38 is used in this configuration, a secondary electron detector, which detects secondary electrons, can also be provided in place of the reflected electron detector.
The electron beam proximity exposure apparatus is controlled by a computer 70. Signals detected by the laser length measuring device 38 for the mask stage and the laser length measuring device 48 for the wafer stage are supplied to a data bus of the computer 70. Signals detected by the reflected electron detector 38, a detector disposed on the standard mark and the height measurer 46 are supplied to a signal processing circuit 76, converted into digital signals and then supplied to the data bus of the computer 70. The condenser lens 18 is an electromagnetic lens or an electrostatic lens, which is controlled by the computer 70 by way of a condenser lens power source 71. The computer 70 supplies deflection amount data to a digital arithmetic circuit 75, which performs an operation to correct the deflection amount data according to previously stored correction data and supplies corrected data to a main DAC/AMP 73 and a subsidiary DAC/AMP 74. The main DAC/AMP 73 and the subsidiary DAC/AMP 74 convert the corrected deflection amount data into analog signals, amplify the analog signals and supply the resulting signals to the main deflecting device 20 and the subsidiary deflecting device 50, respectively. The electron beam is deflected as desired accordingly.
The exposure apparatus described above positions the wafer 40 to the mask and exposes a pattern over an entire surface of the mask by scanning with the electron beam 15.
FIGS. 2(A) and 2(B) are a plan view and a sectional view, respectively, of the mask used in the electron beam proximity exposure apparatus. The mask 30 is a thin plate member with a thickness of a few millimeters, for example, and center portion denoted with a reference number 32 is processed in the thickness of few micrometers, where an aperture pattern is formed in a portion denoted with a reference number 33 within. A reference number 35 denotes a mark for determining a mask's position.
The mask 30 to be used in the above-described electron beam proximity exposure apparatus is a stencil mask having the aperture parts formed as perforations. With the stencil mask, an annular pattern that has a small square pattern 342 in a large square pattern 341 and an aperture as a portion 343 between the large and small square patterns as shown in FIG. 3(A), for example, cannot be exposed with a single mask, and it is necessary to divide this annular pattern into two pairs of patterns 344 and 345 and patterns 346 and 347 as shown in FIGS. 3(B) and 3(C), and expose these patterns in two stages. Such two masks for the exposure of a single pattern are hereafter called complementary masks. In other words, it is necessary to prepare two complementary masks for proximity exposure, perform exposure by the electron beam proximity exposure apparatus in which one (e.g., with the patterns 344 and 345) of the masks is set, and then perform exposure by the electron beam proximity exposure apparatus in which the other mask (e.g., with the patterns 346 and 347) is set. In this case, however, it is necessary after the first exposure to take the wafer out of the vacuum chamber into an atmospheric environment and carry the wafer again into the vacuum chamber of the electron beam proximity exposure apparatus for the second exposure. In other words, it is necessary to repeatedly carry the wafer between the vacuum condition and the atmospheric condition. Accordingly, it arises not only a problem of lowered the throughput but also a problem to allow dust and the like to easily adhere, thereby lowering the yield.
In a case of a photomask to be used in an optical light exposure apparatus such as a stepper, a pattern is formed on a glass substrate with a chromium layer or the like, and then a pellicle layer is formed as a protective film on the pattern. Although the surface of the pellicle layer is monitored for dust adhesion and the surface of the pellicle layer is cleaned to remove the dust in a case that dust adheres on a problematic level, the cleaning does not damage the pattern. The surface of the pellicle layer causes defocusing by its thickness and no particular problem occurs so far as the adhering dust consists of small particles.
In contrast, the mask 30 to be used in the above-described electron beam proximity exposure apparatus is required to be the stencil mask having the aperture parts formed as the perforations, on which the above-described pellicle layer cannot be formed. Therefore, dust or the like adhering to the surface of the mask causes a serious problem in the above-described electron beam proximity exposure apparatus. In a case that the mask is deteriorated by the adhering dust inside the electron beam proximity exposure apparatus, the surface of the mask is hardly cleaned directly, and an apparatus such as a correcting device or a cleaning device is used to remove the dust. However, since the mask is required to be taken out of the electron beam proximity exposure apparatus once for cleaning and to be then mounted again to the electron beam proximity exposure apparatus, the electron beam proximity exposure apparatus is not useable for the while, which results in the lowered throughput.
Since the mask is arranged in proximity to an object, on which a resist layer is applied, with a distance for example about 50 μm away from each other, the mask easily soils with vapor generated from the resist layer and the soil causes charging-up of the mask, which results in an error of an irradiating position. When the mask is soiled, the mask has to be taken out of the apparatus for cleaning or the surface of the mask has to be cleaned by such as plasma-ashing in which ozone is introduced into the chamber. In both cases, the exposure by the electron beam proximity exposure apparatus has to be stopped once for cleaning the mask, which causes the problem of the lowered throughput.
Furthermore, in a case that the mask's minimum line width is not more than 0.2 μm, the thickness of the mask portion needs to be not more than 0.8 μm. According to the electron beam proximity exposure apparatus disclosed in U.S. Pat. No. 5,831,272, the intensity of the electron beam irradiated to the mask is considerably small and the damage of the mask by the irradiated electron beam is extremely small. In spite of it, the mask that is used for a long time is deteriorated and then has a white defect, and the mask thereby becomes unusable. In such a case, the mask is taken out of the electron beam proximity exposure apparatus, corrected by the correcting device, and then mounted again or a new mask is mounted. In both cases, the exposure by the electron beam proximity exposure apparatus has to be stopped once for correcting or replacing the mask, which causes the problem of the lowered throughput.